1. Field of the Invention
The present invention relates to methods and apparatus for vaporizing cryogens and is more particularly directed to methods and apparatus for vaporizing large quantities of liquid nitrogen by drawing a maximum amount of the required heat from ambient air.
2. State of the Prior Art
Nitrogen gas is used in great quantities in connection with the drilling and extraction of underground oil and gas deposits, among many other applications. Such drilling often takes place in remote areas where power is only available from electrical generators at the drilling site. This is particularly true of ocean floor drilling where platforms are anchored far away from land power lines or other sources of energy necessary for the vaporization of large volumes of nitrogen used in the oil drilling and extraction process.
One approach to minimizing the energy input to a nitrogen vaporizer is to utilize the heat available from ambient air. This can be accomplished by passing the liquid nitrogen through a suitably constructed heat exchanger, e.g., a length of tubing provided with fins extending therefrom. In Such devices the tubing is in direct contact with the liquid nitrogen which is at a temperature of minus 320.degree. F. As a result, humidity present in the air condenses on the very cold outer surface of the heat exchanger fins and freezes in the form of a layer of frost covering the heat exchanger surface. This covering of frost can build-up rapidly so that within a short period of time, e.g., 15 to 20 minutes, the performance of the vaporizer is seriously degraded. This type of vaporizer is nonetheless useful in applications where relatively small volumes of gas are required from time to time. While air heated large volume nitrogen vaporizers of the direct contact type have been built, i.e., where the cryogen carrying conduits are directly heated by ambient air through suitable heat exchanger fins or the like, the size of the required heat exchanger structures makes such vaporizers too large for convenient transport and use in many applications.
Therefore, for longer periods of use or greater volumes of gas in ambient air heated vaporizers, it has been necessary to resort to heating devices such as electrical resistance heaters for melting the frost on the heat exchanger to enable continued operation of the vaporizer. This is a brute force approach to overcoming the basic shortcoming of the vaporizer and is wasteful of energy. The energy cost of such a vaporizer where large volumes of nitrogen are required as in oil drilling, are prohibitive for locations such as ocean drilling platforms, particularly in severe climates such as the North Sea.
While other approaches to the problem of vaporizing large quantities of liquid nitrogen are known, such as the use of the heat generated by diesel engines, boilers, etc . . . , no truly effective and efficient means is known for constructing and operating a large volume vaporizer of reasonable physical size with a limited amount of electrical power such as may be available from a generator on an ocean drilling platform while relying primarily on ambient air heating.